This invention relates to surface acoustic wave (SAW) oscillators and in particular to digital compensation for temperature related frequency changes in such devices.
The generation of precise frequencies by means of crystal controlled oscillators is an important electronics function. Recently, SAW oscillators have shown themselves to be a particularly cost effective implementation of this function. The SAW oscillator generally comprises a SAW substrate having input and output transducers on its surface acoustic wave propagating surface. The transducers are positioned to provide a delay path that coincides with a low temperature coefficient of delay orientation. An amplifier connected between the input and output transducers completes, and provides power for, the oscillator circuit.
The two main advantages of SAW devices over traditional bulk crystals are: (1) photolithographic fabrication on a single surface which is applicable to mass production, and (2) straightforward realization of devices operating directly at frequencies of 100-2000 MHz without the multipliers and filters necessary with bulk devices.
A problem associated with the generation of precise frequencies which is shared by both bulk and SAW devices as well as other crystal controlled oscillators is maintaining frequency stability over a wide temperature range. That is, it is desirable to overcome the inherent temperature sensitivity in the crystal element which results in a variation of the frequency of oscillation with temperature.
In the past ovens have been used to maintain the oscillator at a constant temperature. This however adds significantly to the complexity, power requirements, cost and weight of the system in which the oscillator is used. The warm up time required to get the system operational in adverse climates and the additional weight of the oven makes this solution undesirable for many military and airborne applications.
Various other techniques have been proposed to compensate for the temperature sensitivity of SAW oscillators. None of these techniques are capable of very precise compensation, however. In general, many systems require better performance than is possible with the state-of-the-art approach and finer control of the compensation is desired.
Although some improvement is achieved by precise delay path orientation and carefully controlled fabrication processes, the concommitant increase in manufacturing expense and delay times is objectionable.
Accordingly there currently exists the need for a simple, inexpensive and effective means for providing SAW oscillator temperature compensation. The present invention is directed toward satisfying that need.